1. Field of the invention:
The present invention relates to a grating coupler for inputting light into an optical waveguide or emitting light from an optical waveguide in integrated optical elements or other optical elements with the optical waveguide in which light is propagated.
2. Description of the prior art:
Integrated optical pickups, integrated optical scanner elements, integrated optical Doppler speedometers and other integrated optical elements have attained high performance by using an optical waveguide in which light is propagated. In order to input a light into the optical waveguide of the these types of integrated optical elements, the end of said optical waveguide is optically polished and the light is converged by a lens with a large numerical aperture (NA) and input from the optically polished end of the optical waveguide into the inside of the optical waveguide. However, when a light is input into an optical waveguide in this manner, the end of the optical waveguide must be optically polished with high precision and the optical axis of the lens and the optical waveguide must be adjusted precisely.
In contrast to this method, other methods are being widely used whereby light is input into an optical waveguide or emitted from the optical waveguide by means of a grating coupler which is easy to integrate because of its small size and thin structure.
A grating coupler is constituted by positioning a grating on the top of an optical waveguide. The grating has various configurations such as multiple straight lines with the same pitch, multiple curved lines whose pitch gradually changes, and the like.
FIG. 5a is a plan view showing a conventional grating coupler and FIG. 5b is a cross section of the same. The grating coupler is constituted by forming a grating on the top of an optical waveguide 42 that is formed on the top of a crystalline substrate 41 of LiNbO.sub.3 or other material. The optical waveguide 42 is formed in the middle of the substrate from side to side and along its length with a uniform width and thickness. The grating 43 is composed of multiple plate-like transparent elements of equal length and intersecting the optical waveguide 42 perpendicularly to its length. When light is projected on the grating 43, the light enters into the optical waveguide 42 through the grating 43 and is propagated within said optical waveguide 42. The guided light propagated in the optical waveguide 42 is emitted from the grating 43 to the outside of the waveguide 42.
When the guided light 21 propagated in the optical waveguide 42 is emitted from the grating 43 in such a conventional grating coupler, the coupling efficiency is as follows.
The light propagated in the grating coupler gradually attenuates during propagation. Where z represents the coordinate of the direction of propagation in the grating coupler of the guided light 21 in the optical waveguide 42, the intensity of the light emitted from the grating coupler is obtained by the differential equation shown in equation 1. ##EQU1## From equation 1 EQU P.sub.0 .about.exp(-.eta.z) (2)
Accordingly, the intensity distribution of the emitted light is represented by an exponential function as is indicated in FIG. 5b.
In this way, the light propagated in the optical waveguide 42 is emitted from the grating coupler with an exponential intensity distribution. However, since the grating is an optically reciprocal element, if the light entering the grating has this kind of exponential intensity distribution, the intensity of light propagated in the optical waveguide 42 becomes constant, so the coupling efficiency of the grating optical coupler is markedly improved.
In reality, however, it is difficult to give the light entering the grating coupler or propagated in the optical waveguide the exponential intensity distribution explained above, and normally it has a symmetrical intensity distribution like that of a semiconductor laser beam. Therefore, the coupling efficiency of grating couplers is limited to about 80%.